Add GEM MLSum to Full Benchmark

[epavlick]

Spanish/German (including COVID challenge sets)

[Shashi456]

Just spanish and german? The english part of MLSUM is CNN/DM i think, which is the standard summarization dataset

[jzf2101]

See bigscience-workshop/promptsource#738 for pr - I am reviewing this PR but will need additional help from speakers of German and Spanish for the prompts